1. Field of the Invention
This invention relates to the stabilization of pyrolysis gasoline (xe2x80x9cpygasxe2x80x9d), and more particularly to lengthening the life-time cycle of first stage hydrogenation of pygas.
2. Description of the Prior Art
Crude oil fractions such as a straight run naphtha from a crude oil still are conventionally steam cracked in a olefins unit to produce light olefins and aromatics, valuable chemicals in their own right. Pygas is a valuable by-product of such steam cracking because it is generally high octane and within the general gasoline boiling range of from about 100 to about 420xc2x0 F., and can be used as a finished gasoline blending stream after undergoing certain processing before blending.
Because pygas is derived from steam cracking complex hydrocarbon streams such as naphthas, it can carry with it a large amount of widely varying catalyst poisons that interfere with the aforesaid pre-blending processing of pygas. The amount and severity of pygas poisons is unusually severe as compared to other gasoline producing streams, e.g., gasolines from catalytic cracking units. This makes pygas pre-blending processing quite detrimental to catalyst life during such processing.
Also unlike other gasoline streams used for finished gasoline blending, pygas, before first stage hydrotreating, contains substantial amounts of gum precursors, and has poor oxidation stability.
Accordingly, pygas is challenging to stabilize and otherwise process before gasoline blending is undertaken.
The first stage of pygas processing before blending is often hydrotreating over a Group VIII metal catalyst (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum) to selectively hydrogenate gum precursors such as diolefins, acetylenics, styrenics, dicyclopentadiene, and the like while not hydrogenating significant amounts of mono-olefins, aromatics, and other gasoline octane enhancers. Competitive adsorption causes diolefins and acetylenics to be hydrogenated preferentially over mono-olefins and aromatics thus removing gum tendencies while maintaining octane value. Paraffins are left unchanged or mildly isomerized which can help gasoline value.
Sometimes several stages of selective hydrogenation are carried out.
Second stage hydrotreating is often done on a BTX (benzene, toluene, and xylenes) fraction of pygas for removal of sulfur and other impurities.
The poison severity usually found in pygas can severely reduce first stage hydrogenation catalyst activity and catalyst life. For example, while sulfur, carbonyls, basic nitrogen, and gums/coking tend to be temporary catalyst poisons, arsenic, mercury, lead, and phosphorous tend to be more permanent poisons. Other permanent poisons include trace silicon oxide and corrosion metal oxide dusts which tend to plug catalyst pores. Also, polysiloxanes thermally decomposed and permanently poison palladium or nickel catalysts.
Guard beds can be employed upstream of a first stage hydrotreater to remove such poisons, but this is an expensive approach, and it is not always physically possible or otherwise practical to install guard beds and regeneration capability.
Thus, it is very desirable to have a pygas first stage hydrogenation catalyst that remains robust as to both selective hydrogenation activity and catalyst life when exposed to the pygas poison severity without resorting to a guard bed or other processing to remove or neutralize poisons before such first stage hydrotreating.